A common trend in modern integrated circuit manufacturing is to produce transistors having very small feature sizes. For competitive high density integrated circuits, features such as the conductors, source and drain junctions, and interconnections to the junctions must be made as small as possible. As feature sizes decrease, the sizes of the resulting transistors and the interconnections between transistors also decrease. Smaller transistors allow more transistors to fit on a single substrate. Furthermore, smaller transistors usually have lower turn on threshold voltages and taster switching speeds and consume less power in their operation. These features allow for higher speed integrated circuits.
As semiconductor transistors become smaller, a number of problems have arisen. For instance, use of a very thin gate dielectric causes high gate current leakage, which diminishes device performance. Also, a higher doping level is needed in the channel to reduce short channel effect in order to ensure that the transistor properly turns off. Using a very high concentration of dopant in the channel decreases current drive and can lead to undesirable drain-to-channel tunneling current.
Polysilicon gate technology, which is often employed, carries with it additional problems. For example, polysilicon gates tend to suffer from polysilicon depletion or boron penetration effects, causing poor performance. Additionally, a polysilicon gate has a fixed work function determined by a certain high level of doping.
Metal is another material used for gate electrodes. Metal has a variety of advantages over polysilicon as a gate electrode material. For example, metal allows for excellent current flow and has less voltage depletion problems than polysilicon. Metal too, however, has its own drawbacks. Some metals, like Ti and Ni, are highly diffusive and act as contaminants within the channel region, particularly during the high temperature conditions required for dopant activation of the source/drain implant. Also, certain work functions are required that allow transistors to work optimally, and it is more difficult to manipulate the work function of metals than it is to manipulate the work function of polysilicon. Moreover, metals are difficult to etch properly. Dry-etch methods are too harsh on underlying Si substrates while wet-etch methods can excessively undercut the sidewalls of the gate electrode.
Methods to solve some of these problems have been attempted by combining the conventional methods of forming the transistor with polysilicon as the gate electrode during doping with the additional steps of completely etching out the polysilicon after doping and replacing it with a metal. This replacement process, however, is complex and can often result in costly errors if not done properly.